Hydraulic fracturing is a well stimulation technique designed to increase the productivity of a well by creating highly conductive fractures or channels in the producing formation surrounding the well. The process normally involves two basic steps: (1) injecting a fluid at sufficient rate and pressure to rupture the formation thereby creating a crack (fracture) in the reservoir rock, and (2) thereafter placing a particulate material (propping agent) in the formation to maintain the fracture walls open by resisting forces tending to close the fracture. If stimulation is to occur, the propping agent must have sufficient mechanical strength to bear the closure stresses and provide relatively high permeability in the propped fracture.
Although a variety of particulate materials are used as proppants, silica sand remains the only material that is used extensively. It has been long recognized, however, that sand is not the ideal proppant. Sand at closure stresses encountered in deep formations tends to disintegrate, producing fines or fragments which can drastically reduce permeability of the propped fracture.
Efforts to develop a proppant capable of bearing high stresses without excessive fragmentation have produced such material as glass beads, steel shot, aluminum pellets, zirconium oxide, sintered bauxite and sintered mullite. Many of the available high strength proppants have not proven satisfactory for deep wells. Multilayers of glass beads, for example, at high stress levels under reservoir conditions do not provide high fracture permeability. Steel shot and aluminum pellets ar susceptible to corrosion by formation brine. Sintered bauxite is susceptible to conversion when treating a formation with HF acid. A sintered bauxite proppant is discussed in U.S. Pat. No. 4,068,718 issued to Cooke et al.
U.S. Pat. No. 3,399,727 discloses the use of propping agent particles composed of ceramics having internal voids. These particles, preferably made by the evolution of gas bubbles within a molten ceramic (e.g. glass, procelain or alumina), are designed to have partial crushability characteristics. According to the patent, the slow settling rate resulting from the low density particles and the partial crushability features make the proppants suitable for partial monolayer propping. However, such particles, because of their cellular structure and partial crushing characteristics, do not have sufficient compressive strength to withstand high stresses encountered in deep well fractures.
Therefore, what is needed is a proppant which can be utilized in hydraulic fracturing which once in place can withstand high stresses encountered in deep wells and acid conditions used in treating a formation.